1. Field of the Invention
The present invention relates to a leak diagnosis device for performing leak diagnosis of determining whether there is a leak hole in a purge device, which adsorbs fuel vapor generated in a fuel tank with an adsorbent in a canister and purges the fuel vapor adsorbed by the adsorbent into an intake passage of an internal combustion engine.
2. Description of Related Art
The purge device inhibits diffusion of fuel vapor generated in a fuel tank to an atmosphere. The purge device introduces the fuel vapor in the fuel tank into a canister accommodating an adsorbent and temporarily adsorbs the fuel vapor with the adsorbent. The vapor fuel adsorbed by the adsorbent separates from the adsorbent due to a negative pressure generated in an intake pipe during operation of an internal combustion engine and is discharged (purged) to the intake pipe of the engine through a purge passage.
In such the purge device, if a leak hole is formed in a passage introducing the fuel vapor to the intake pipe of the engine, a canister or the like, the fuel vapor is discharged to the atmosphere through the leak hole. If the leak hole is formed in the purge device, the leak hole should be detected as early as possible. Therefore, for example, a leak diagnosis device described in JP-A-2004-293438 detects the pressure in the purge device at the time when the pressure in the purge device is decreased or increased. The leak diagnosis device performs leak diagnosis of existence or nonexistence of the leak hole in the purge device based on the magnitude of the pressure or the pressure change.
Since the leak diagnosis device diagnoses the existence or nonexistence of the leak hole by detecting the pressure in the purge device, it is difficult to perform the diagnosis accurately under a condition that the pressure in the purge device changes easily, e.g., under a condition that the pressure in the fuel tank changes because of shaking of the fuel in the fuel tank or a change in atmospheric pressure at the time when a vehicle is running on an upslope. Therefore, the diagnosis device described in JP-A-2004-293438 performs the diagnosis when the pressure in the purge device stabilizes, i.e., during an idling state or after the engine is stopped.
However, a large amount of the fuel vapor is generated immediately after the engine is stopped because the fuel temperature has increased due to heat generation from a fuel pump provided in the fuel tank, for example. Accordingly, the pressure in the purge device is unstable. Therefore, the leak diagnosis after the stopping of the engine is performed when a predetermined period necessary for the pressure in the purge device to stabilize elapses.
However, practically, the period necessary for stabilizing the pressure in the purge device and for providing a suitable condition for the leak diagnosis fluctuates due to influences of an environment in which the vehicle is located (e.g., ambient temperature, solar radiation, radiant heat from ground or wind). Therefore, in the case where the leak diagnosis is performed when the predetermined period elapses after the engine is stopped, the predetermined period has to be set sufficiently long in order to ensure the accuracy of the leak diagnosis even under a certain environmental condition that requires the longest period for the pressure in the purge device to stabilize. Thus, the predetermined period has to be set relatively long. Accordingly, there is a great possibility that the engine is restarted before the predetermined period elapses after the engine is stopped and that the opportunity of the leak diagnosis is reduced.